The present invention refers to a knee prosthesis comprising two pivotally interconnected members carrying a locking device and a first and a second axle, said first axle forming a bearing axle for the locking device and the second axle cooperates with the locking device for activation thereof said locking device is arranged to permit said members to pivot in an unloaded position but prevent pivoting from extended to bent position when loaded.
Traditional friction locks in knee prostheses will lock when there is a load acting on them, i.e. when some part of the body weight rests on the artificial knee. This takes place in a still standing position as well during the walking phase at heel strike when the heel hits the ground, and during toe off supporting oneself on the toe while extending the leg to initiate the swing phase when the leg swings freely in the air.
At normal walking without a prosthesis one starts already during toe off to flex the knee-joint to initiate the swing phase fore all body weight is has been removed from the leg in question. This is not possible with knee prostheses with a friction lock of the conventional kind. This involves an unnatural walking and makes walking in stairs and broken ground and cycling difficult since the knee prosthesis will lock as soon as it is loaded.
Knee prostheses with friction locks in the form of a brake drum are disclosed in U.S. Pat. Nos. 4,206,519 and 4,351,070. In the last mentioned document there is a linkage transferring motions in the hip axis to the locking device in order to control the locking function thereof in response to the torque of the hip axis. PCT Publication WO 97/10781 discloses a knee prosthesis with a friction lock in the form of a resiliently deformable substantially C-shaped member cooperating with an axle. The C-shaped member can be rotated about the axle in unloaded position, but in a locked position be deformed and locked to the axle.
The present invention provides a knee prosthesis, which permits rotation of the knee-joint in an unloaded position and when it still is under a body load during toe off when supporting oneself on the toe while extending the leg to initiate the swing phase, but which locks against rotation from extended to flexed position at other loaded positions, for example during heel strike when supporting oneself on the heel and during mid stance when supporting oneself on the whole foot. The present invention also provides a geometric sensitivity of the locking device, due to the fact that the bearing axle of the locking device, said first axle, is arranged out of center of the locking device, so that depending on where a load line from a load on the knee-joint passes through it with respect to its geometrical center and with respect to said first axle, the rotating part of the locking device will either be in balance, at which it is unactivated, or out of balance, at which it will be activated.
A further important advantage of the invention is that the knee-joint has a freewheel effect in such a way that it can always rotate from flexed to extended position also under load. By this, for example walking on stairs will be possible. The knee-joint will however lock immediately again if it is rotated in the opposite direction, for example from extended to flexed position, under such load that the locking device is activated.